There are several modern methods for fabricating optical waveguides for the low-loss containment and delivery of optical waves. One such waveguide is optical fiber which slightly higher index of refraction than the surrounding cladding. Typical values for the core diameter are of order 10 xcexcm for single-mode fiber operating at communications wavelengths of 1300-1550 nm, and 50 xcexcm or 62.5 xcexcm for highly multi-mode fiber. Whether single-mode or multi-mode, the cladding diameter has most commonly an overall diameter of 125 xcexcm, and a plastic jacket diameter is typically 250 xcexcm for standard telecommunications fiber. The glass core is generally doped with germanium to achieve a slightly higher index of refraction than the surrounding cladding by a factor of roughly 1.003. The jacket is generally plastic and is used to protect the core and cladding elements. It also presents an optically discontinuous interface to the cladding thereby preventing coupling modes in the cladding to other adjacent fibers, and usually plays no significant part in the optical behavior of the individual fiber other than the usually rapid attenuation of cladding modes in comparison with bound core modes.
As described in the book by Snyder and Love entitled xe2x80x9cOptical Waveguide Theoryxe2x80x9d published by Chapman and Hall (London, 1983), under the assumptions of longitudinal invariance and small index differences for which the scalar wave equation is applicable, the modal field magnitudes may be written
xcexa8(r, xcfx86, z)=xcexa8(r, xcfx86) exp{i(xcex2zxe2x88x92xcfx89t)}
where
xcex2 is the propagation constant
xcfx89 is the angular frequency
t is time
z is the axial distance
r, xcfx86 is the polar trans-axial position along the fiber.
Single-mode fibers support just one order of bound mode known as the fundamental-mode which we denote as xcexa801, and which is often referred to in the literature as LP01. The transverse field dependence for the fundamental-mode in the vicinity of the core may be approximated by a gaussian function as
xcexa801(r, xcfx86)=exp{xe2x88x92(r/r01)2}
where r01 is the fundamental-mode spot size.
Optical fiber couplers, also known as power splitters, are well known in the art, and generally comprise two fibers as described above having their jackets removed and bonded together with claddings reduced so as to place the fiber cores in close axial proximity such that energy from the core of one fiber couples into the core of the adjacent fiber. One such coupler is a fused coupler, fabricated by placing two fibers in close proximity, and heating and drawing them. The finished fused coupler has the two cores in close proximity, enabling the coupling of wave energy from one fiber to the other. A further subclass of fused coupler involves a substantially longer coupling length, and is known as a wavelength discriminator. The characteristics of a wavelength discriminator include wavelength-selective coupling from an input port to a first output port, as well as a second output port. As the wavelength is changed over the operating range of the wavelength discriminator, more energy is coupled into the first output port, and less is coupled into the second output port. The operation of a wavelength discriminator is described in xe2x80x9cAll-fibre grating strain-sensor demodulation technique using a wavelength division couplerxe2x80x9d by Davis and Kersey in Electronics Letters, Jan. 6, 1994, Vol. 30 No. 1.
Fiber optic filters are well known in the art, and may be constructed using a combination of optical fiber and gratings. Using fiber of the previously described type, there are several techniques for creating fiber optic gratings. The earliest type of fiber grating-based filters involved gratings external to the fiber core, which were placed in the vicinity of the cladding as described in the publication xe2x80x9cA single mode fiber evanescent grating reflectorxe2x80x9d by Sorin and Shaw in the Journal of Lightwave Technology LT-3:1041-1045 (1985), and in the U.S. patents by Sorin U.S. Pat. No. 4,986,624, Schmadel U.S. Pat. No. 4,268,116, and Ishikawa U.S. Pat. No. 4,622,663. All of these disclose periodic gratings which operate in the evanescent cladding area proximal to the core of the fiber, yet maintain a separation from the core. A second class of filters involve internal gratings fabricated within the optical fiber itself. One technique involves the creation of an in-fiber grating through the introduction of modulations of core refractive index, wherein these modulations are placed along periodic spatial intervals for the duration of the filter. In-core fiber gratings were discovered by Hill et al and published as xe2x80x9cPhotosensitivity in optical fiber waveguides: Application to reflected filter fabrication,xe2x80x9d in Applied Physics Letters 32:647-649 (1978). These gratings were written internally by interfering two counter propagating electromagnetic waves within the fiber core, one of which was produced from reflection of the first from the fiber end face. However, in-core gratings remained a curiosity until the work of Meltz et al in the late 1980s, who showed how to write them externally by the split-interferometer method involving side-illumination of the fiber core by two interfering beams produced by a laser as described in the publication xe2x80x9cFormation of Bragg gratings in optical fibers by a transverse holographic methodxe2x80x9d in Optics Letters 14:823-825 (1989). U.S. patents Digiovanni U.S. Pat. No. 5,237,576 and Glenn U.S. Pat. No. 5,048,913, also disclose Bragg gratings, a class of grating for which the grating structure comprises a periodic modulation of the index of refraction over the extent of the grating. Short-period gratings reflect the filtered wavelength into a counter-propagating mode, and, for silica based optical fibers, have refractive index modulations with periodicity on the order of a third of the wavelength being filtered. Long-period gratings have this modulation period much longer than the filtered wavelength, and convert the energy of one mode into another mode propagating in the same direction, i.e., a co-propagating mode, as described in the publication xe2x80x9cEfficient mode conversion in telecommunication fibre using externally written gratingsxe2x80x9d by Hill et al in Electronics Letters 26:1270-1272 (1990). The grating comprises a periodic variation in the index of refraction in the principal axis of the core of the fiber, such variation comprising a modulation on the order of 0.1% of the refractive index of the core, and having a period associated with either short or long-period gratings, as will be described later.
The use of fiber-optics in temperature measurement is disclosed in U.S. Pat. No. 5,015,943 by Mako et al. A laser source is beam split into two fibers, one of which is a sensing fiber exposed to an elevated temperature, and one of which is a reference fiber in an ambient environment. The optical energy from the two fibers is summed together, and an interference pattern results. As the temperature changes, the physical length of the sensing fiber optic cable changes, which causes the interference pattern to modulate. Each modulation cycle represents one wavelength change in length. Counting these interference patterns over time enables the measurement of temperature change.
The present invention is directed to an apparatus for the measurement of sensor grating pitch, wherein the change in grating pitch can originate from a strain applied to the sensor grating, or it may originate from a temperature change wherein the sensor grating expands or contracts due to the coefficient of thermal expansion of the optical fiber enclosing the sensor grating. A pair of fibers, each having a sensor grating, is illuminated by a pair of broadband sources coupled through a pair of optical power splitters, and this sensor grating reflects wave energy over a narrow optical bandwidth. Reflected wave energy from the narrow-band sensor grating is passed through a wavelength discriminator, comprising a long-drawn optical coupler. A normalized power ratio comprises the difference in first and second detector power levels divided by the sum of the first and second power level. This intensity ratio is compared to the wavelength discriminator characteristic stored in a controller to look up the wavelength from a normalized power ratio value, and hence the pitch of the sensor grating. As the characteristic of the wavelength discriminator is essentially temperature invariant, this very accurately yields the sensor grating pitch. Comparing this reflected wavelength to the known wavelength of the grating indicates a change in wavelength brought about by either a temperature change or by the presence of a strain. In the case where a second sensor is also monitored, one sensor may be used as a reference to monitor the temperature of the second sensor, which is used to measure applied strain. In this manner, the temperature effect of the strain gauge may be cancelled by using the measured result of the reference sensor. Commutating the two sources in separate non-overlapping intervals enables the independent measurement of temperature, or strain, or any combination of the two.